STEVE INSKEEP, Host:
Here's one less thing to worry about in a stressful time: Earth's risk of being hit by a giant rock from outer space may be smaller than scientists previously thought. That's according to the most accurate census yet of near-Earth asteroids. NPR's Nell Greenfieldboyce reports that a NASA space telescope recently scanned the skies, searching for asteroids lurking nearby and found far fewer than astronomers had expected.
NELL GREENFIELDBOYCE: The Earth has been whacked by big space rocks in the past. One about six miles across is thought to have wiped out the dinosaurs. Scientists would like to prevent something like that from happening in the future. Amy Mainzer is at NASA's Jet Propulsion Lab in California.
AMY MAINZER: As one of my colleagues at the jet propulsion laboratory likes to say, the best three ways of dealing with the potential of an asteroid impact are to find them early, find them early, and find them early.
GREENFIELDBOYCE: Because an early warning would mean time to figure out how to knock the asteroid off a collision course.
To see how many potentially dangerous asteroids might actually be out there, Mainzer and her colleagues did a new survey. Most of the known near-Earth asteroids have been discovered with ground-based telescopes, but these can't see everything. So her team used an infrared space telescope that was launched in 2009 to get a more reliable representative sample of these asteroids that orbit the sun and have a risk of crossing Earth's orbit.
MAINZER: Our understanding of the near-Earth asteroid population has been significantly improved, and we believe that the hazard to the Earth may be somewhat less.
GREENFIELDBOYCE: Mainzer says the study suggests that astronomers already know the location of more than 90 percent of the very largest asteroids, the huge planet-busters that could cause mass extinctions.
MAINZER: By virtue of the fact that we know these objects and we know their orbits, we can predict that they are no longer hazardous to Earth, in the sense that we can follow them and we know that there are none that pose any imminent risk of an impact.
GREENFIELDBOYCE: There's also good news when it comes to mid-size asteroids near Earth. The survey suggests there are only about 19,000 of them. Mainzer says that's far fewer than the 35,000 or so that scientists had expected would be out there.
MAINZER: However, it's very important to note that fewer does not mean none.
GREENFIELDBOYCE: And scientists don't know where most of those are. There are an estimated 15,000 mid-sized asteroids left to locate. Lindley Johnson heads the near-Earth object program at NASA headquarters in Washington, D.C. He says an asteroid the size of a football field could wipe out a major city.
LINDLEY JOHNSON: If, say, for instance, one were to hit in the middle of the D.C. area, it would pretty much devastate the entire area within the Beltway.
GREENFIELDBOYCE: And that's why NASA needs to keep searching.
JOHNSON: We continue to run several ground-based teams that have been in operation for several years and have actually found the majority of the known objects.
GREENFIELDBOYCE: Hundreds of mid-sized asteroids near Earth are discovered every year, but at this rate it would take decades to locate all of them and see if they pose a threat.
Last year, an expert committee convened by the National Research Council said there was no way NASA could meet a 2020 deadline set by Congress to find 90 percent of the big and mid-sized asteroids. It noted that NASA's budget for this kind of work has historically been small - only about $4 million a year. Johnson says the administration recently requested a budget increase to about 20 million. But that's still for Congress to decide.
Nell Greenfieldboyce. NPR News.